The present invention pertains to an electrochemical gas sensor with at least two electrodes, a diffusion membrane and an electrolyte, and relates to a process for making same.
Such a gas sensor is shown in DE 43 35 409 C2, where the housing jacket surrounding the electrolyte space is designed as a barrier, which is permeable to gases with respect to the environment but is impermeable to the electrolyte and via which pressure equalization takes place between the electrolyte space and the environment without the electrolyte being able to run out. In this gas sensor and in other prior-art gas sensors, the electrodes used are introduced into the sensor housing separately and individually and are contacted by means of suitable wires or pins, and the electric contacts are led to the outside through the sensor housing. The electrodes are arranged stacked at spaced locations from one another. Special separators in the form of, e.g., porous, electrolyte-impregnated glass mats are used for this purpose, so that no electric short-circuit can develop between the electrodes. These prior-art electrochemical gas sensors require a great, predominantly manual effort for their assembly, which is, moreover, complicated and may lead to errors.
The prior-art gas sensor of this type with a gas-permeable housing offers the following advantages in practice: It guarantees a position-independent pressure equalization between the interior space of the sensor and the environment, high chemical resistance as well as very good tightness properties, as a consequence of which the measurement of the concentration of the measured gas or measured gases is highly reliable, and the service life is long.
It is an object of the present invention to provide a gas sensor with the known properties, which has a more compact design in terms of size and can be manufactured with a reduced manufacturing effort and consequently at a lower cost.
According to the invention, an electrochemical gas sensor is provided with at least two electrodes, a diffusion membrane and an electrolyte. The gas sensor has a conical, hemispherical or cylindrical outer housing with gas admission openings and with a cover. A layer structure is provided comprising, from the outside to the inside, the diffusion membrane with the electrodes applied thereto in a planiform manner, a layer of a mat material or a porous body, which accommodates the electrolyte, and an electrolyte space filled at least partially with the electrolyte.
An advantage of the present invention is the use of a nonplanar diffusion membrane and of a nonplanar electrode array in the gas sensor. All electrodes, i.e., the measuring electrode, the reference electrode and optionally the auxiliary electrode, are first applied alternatively, optionally with the associated strip conductors or contact paths, either to a cone envelope-shaped or circular, planar diffusion membrane or to a rectangular, likewise planar diffusion membrane. The cone envelope-shaped or circular diffusion membrane is subsequently welded into a rounded cone envelope-shaped or calotte-shaped or hemisphere-shaped housing part, and the rectangular diffusion membrane is alternatively welded into a cylindrical housing part. The electrodes are covered with a mat that is absorbent with respect to the electrolyte or with a porous body. Electrolyte is subsequently filled into the rounded cone envelope-shaped or calotte-shaped or hemisphere-shaped or alternatively cylindrical array, and the housing is closed with a flat cover. The contacting with a plug for the connection to an external evaluating unit is performed from the outside via the strip conductors or the contact paths. The leakage of electrolyte through this opening for the electric contacting can be reliably prevented from occurring by means of a suitable, prior-art sealing material. Another essential advantage of the present invention arises from the fact that the diffusion membrane accommodating the electrodes and their electric lines can be manufactured in an automated manner at low cost, so that the complicated and time-consuming assembly of the individual components, which has hitherto been usual, is considerably simplified.
More than one measuring electrode may be provided for the simultaneous measurement of more than one measured gas with a said measured gas-specific measuring electrode for each.
The gas admission openings may advantageously be located in the area of the maximum distance from the cover. The housing with the cover may consist of a gas-impermeable material, especially polypropylene, polyethylene, polystyrene, polycarbonate, PMMA (polymethyl methacrylate), PSU (polysulfone), FEP (copolymer from hexafluoropropylene and tetrafluoropropylene), or PFA (perfluoroalkoxy polymer). The housing may be designed in the form of a hemisphere or a cylinder with semicircular or U-shaped cross-sectional area.
The mat material may be a polymer material, glass, quartz or ceramic fibers, and the porous body may be formed of the same materials.
Electric lines associated with the electrodes may be applied to the diffusion membrane in the form of contact paths or strip conductors. The electrodes and/or the electric lines may be printed, sintered, sputtered or vapor-deposited on the said diffusion membrane.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.